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Ensuring the Durability of Energy -Efficient Houses Why are so many of today's homes plagued by rot, mildew and stale ai r, and what can we do about it? BY STEPHEN SMULSKI expensive and very likely the least durable houses ever built in the United States. Over the past 20 years, the frequency of moisture-related problems in new houses has skyrocketed. Frustrated homeowners complain increasingly of window condensation, mold and mildew indoors; of extractive staining and peeling paint outdoors (top photo); and of rotting windows, doors, trim (bottom photo, facing page), siding, sheathing and framing. All these problems are occurring within a few years of construction. T Tighter walls, colder walls Historically, moisture-related problems were uncommon; the architects, builders and occupants of wood-frame houses relied-perhaps unwittingly-on the natural replacement of air to control indoor humidity and to keep walls dry. Differences in temperature and pressure across a house's envelope provided the driving force for moving air through random leaks in its walls, foundation and attic. As a result, warm, moist indoor air was flushed out and replaced with cooler (and usually drier) outdoor air. Reliance on the natural replacement of air by way of random leakage worked fine for he energy-efficient wood-frame houses of the 1980s and '90s are the most comfortable, the most many centuries until the rapid evolution of technology in the 20th century made it possible to build houses that had tighter and tighter envelopes. Insulation was first added to walls on a large scale in the 1920s. Functioning as a physical barrier that hindered movement of air and conduction of heat through the wall, the insulation often lowered the temperature of the interior side of the sheathing below the dew point during cold weather. Mildew and mold sometimes appeared on the back of the sheathing as a result of condensation from warm, moist indoor air that escaped through gaps in the insulation. To improve the thermal performance of insulation and to protect it from rain seepage, building papers were soon being applied over the sheathing, further reducing airflow through the walls. The trend toward building tighter, and therefore colder, walls continued through the 1940s and 1950s as plaster was replaced by drywall and as lumber sheathing gave way to insulation board and plywood. The introduction in the 1960s and beyond of electric heat and low-draft furnaces meant that large volumes of moist air were no longer being expelled through an active chimney. In the 1970s and '80s, widespread adoption Paint problems. To prevent premature paint failure, exterior wood products should be backprimed, end-primed and top-coated within two weeks with two coats of a highpermeability paint or solid-color stain.